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21 Factsheet 10: Waves

Waves around New Zealand’s open coast derive from two sources:

  • locally generated waves caused by local winds
  • distantly generated (swell) waves formed within the wider Pacific Ocean or Southern Ocean.

Waves tend to be defined by their significant wave height (Hs), which is the average height of the highest 33% of waves over a certain period; the wave period (Tm), which is the average time between successive waves; and the wave direction.

Offshore wave conditions around New Zealand can be subdivided into four major zones in terms of open-coast wave exposure:

  • south-facing coasts (Fiordland to Catlins, South Island): an extremely high-energy wave zone (mean Hs = 3–4 m; Tm = 10–12 s; SW–W).  Waves are typically steep, indicating a zone of active wave generation, but also contain a sizable swell component from the Southern Ocean
  • western New Zealand coasts: a high-energy wave zone (mean Hs = 2–3 m; Tm = 6–8 s; SW–W).  The waves are steep and respond to the regular passage of weather systems across the Tasman Sea
  • eastern New Zealand, up to East Cape: a moderate to high-energy wave zone (mean Hs = 1.5–3 m; Tm = 6–9 s; S).  Sheltered from prevailing westerly winds by the New Zealand landmass but exposed to southerly winds and swell.  Wave steepness is variable, indicating a mixed swell and local sea
  • northeastern North Island (East Cape to North Cape): a low –energy, lee shore (mean Hs =1–2 m; Tm= 5–7 s, N–E).  Wave steepness is variable.  Highest waves occur during ex-tropical cyclones, or as swell that is generated by Pacific cyclones well out to the northeast of the North Island.
 20-year average of the significant wave height (Hs)

20-year average of the significant wave height (Hs) around New Zealand, based on a deep-water wave model. Note: results are only approximate in coastal areas.

In estuaries and harbours, waves are mostly generated by local winds and their height is limited by the wind fetch and the depth of water.  Fetch is the distance downwind of continuous open water, with long fetches allowing the wind to build up larger waves.  Wind waves in estuaries and harbours can still cause erosion and inundation hazards, particularly during very high tides or storm tides.

Very little monitoring of wave conditions has been carried out around New Zealand.  Consequently, to assess wave climate and derive probabilities of extreme wave conditions, use is made of computer models to hindcast wave conditions from past wind conditions over a sufficient period of time (decades).  Two types of model are typically used:

  • deepwater wave models that simulate oceanic wave conditions over a large part of the Southern and Southwest Pacific oceans (right, upper figure) based on global wind fields
  • nearshore wave models that simulate the changes in deepwater wave conditions as the waves approach the shore brought about by wave refraction, diffraction and shoaling.  These models cover a small regional area and are driven by deepwater wave conditions on the offshore boundary and local winds over the region being modelled (eg, right, lower figure for the Bay of Plenty).

• deepwater wave models• nearshore wave models

Text description of left figure: An example of simulated oceanic wave heights for 12 o’clock on the 1 July 1998 over the Southern Ocean from west of Australia to the mid-Pacific Ocean and from Antarctica to north of Australia.

Text description of right figure: An example of simulated wave conditions along the Bay of Plenty coastline from Waihi to East Cape for an offshore significant wave height of 2 metres, wave period of 10 seconds, wave direction of 22.5 degrees North and local wind speed of 5 metres per second from the north-west.

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